Sound recording apparatus



June23, 1942. 4 E. w. KE,LLOYGG 2,287,253.

SOUND RECORDING APPARATUS Filed Oct. 17, 1940 2 Sheets-Sheet 2Edward/[112110 Gttorneg Patented June 23, 1.942

SOUND RECORDHVG APPARATUS Edward W. Kellogg, Moorestown, N. 3., assignorto Radio Corporation America, a' corporation of Delaware ApplicationOctober 17, 1940, Serial No. 361,560 Claims. (Cl.1?79100.3)

This invention relates to improved ground noise reduction apparatus and,more particular ly, to an amplifier circuit which is adapted to respondto electrical impulses corresponding to sound impulses, in such a manneras tov provide the most desirable ground noise reduction characteristicsin, recording. This amplifier provides a sufilciently rapid response tothe-sound impulses without either providing so much clearance as toproduce undue ground noise or clipping the peaks of the sound waverecords.

Although the apparatus hereinafter described This. prior art arrangementhad a number of disadvantages which are-overcome by the presentapparatus. In the sound waves which have a plurality of positive peaksbefore any appreciable negative peak amplitude is reached, there wouldbe no appreciable movement of the shutterduring the period of negligiblysmall negative peaks and, when the negative peaks would appear, theymight be of considerable amplitude, thereby causing clipping of a numberof waves. This could be avoided by using full wave rectificationisdescribed with particular reference to its application to.a variablearea sound recording apparatus, it will be understood that it is equallyapplicable to variable density recording appara-' tus and that thedescription in conjunction with variable area apparatus is given for thereason that it is easier to .visualize the operation of the variousparts and the types of records produced.

- When any sound wave is produced, it consists of a series ofcompressions and rarefactions of the air. In the compressions; thepressure and the air. density are, of course, greater than the normalatmospheric pressure, and the reverse is true of the rarefactions.Accordingly, '1 hereinafter refer to the compressive peaks of the soundwaves as positive pea! and to the peaks of rare- 0 faction as negativepeaks.

As was pointed out in my-paper entitled Starting characteristics ofspeech sounds, published in the Journal of the Society of Motion PictureEngineers, volume XXXIV, page 43,

there are often positive peaks of appreciable -magnitude considerably inadvance of thenega tive peaks of the same sounds, and the positive peaksin a continued sound-frequently exceed thenegative peaks in amplitude,while the reverse is rarely the case.

Due to the characteristics of sounds, it is customary inthe art toconnect the recording galvanometer light valve or otherlight-controlling device to theamplifier in such a manner that thedirection or greatest permissible swing is in the same direction as thepositive peaks of the sound waves.

When ground noise reduction apparatus is used with such sound-recordingapparatus, due to the characteristics of the lightcontrolling device thepolarity is such that the positive peaks are directed away from theground noise reduction control and the ground noise re-.

duction apparatus follows the general contour of the negative peaks. Asa'result of this, the

movement of the ground noise reduction apparatus is kept to a minimum.

but the use of full wave rectification is undesirable for the reasonthat the amplitude of the positive peaks is in many instances so muchhigher than that of the negative peaks of the average that too muchmargin would be produced.

onthe negative side of the record. It is desirable, however, to usea-portion oi the amplitude of the positive peaks to cause initialopening of the ground noise reduction apparatus, but to limit theefiective amplitude so that it cannot greatly exceed the averageamplitude of the negative peaks. During a crescendo, each successivepeak is, of course, higher than the preceding one, and, in this case, ifa portion-of the energy of the positive peaks can be added to that ofthe negative peaks, it will considerably assist in preventing clipping.

Since the usual ground noise reduction circuit is of the peak-responsivetype which opens in .response to the peaks and proportionally to theheight of the peaks and thereafter gradually closes, the addition of aportion of the positive peaks equal inheight to'the negative peaks willassist in preventing clipping merely due to the shorter -intervalbetween peaks. The positive peaks willassist still more in preventingclipping during crescendos, even though they be limited in amplitude,because in so many instances the positive peaks come first. If thepositive peaks as applied to the ground noise reduction system arelimited to a value less than that of the negative peaks, -as, forexample, when the latter are of moderately high amplitude, the positivepeaks may still advantageously be included in.-

sucha manner that they provide a limited marin of response ofthe groundnoise reduction system above that produced by the negative peaks alone.Such-margin will takezcare of a, moderateunexpected increase in theheight of the negative peaks. Therefore, in order to obtain the benefitsjust described, in accordance with my present invention, I employ acircuit which limits the magnitude of the positive peaks applied to theground noise reduction system,

and cause these peaks to contribute a limited amountto the totalresponse of the ground noise reduction system.

' One object of my invention is to provide an improved ground noisereduction circuit.

Another object of my invention is to provide an improved type ofrectifying amplifier.

Another object of the invention is to provide an improved form ofpeak-reading circuit. I

Another object of the invention is-to provide a ground noise reductionapparatus for use in sound recording which will respond with reasonableaccuracy to the envelope of the waves.

Another object of the invention is to provide a ground noise reductioncircuit in which the impulses due to the negative wave peaks arecombined with a certain portion of the impulses due to the positivewave. peaks.

Another object-of the invention is to provide a ground noise reductioncircuit in which impulses due to the positive wave peaks are comcessgain is normally bined with impulses from the negative wave peaks up tobut not materially exceeding the amplitude of the negative wave peaks.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from a reading of the following specification and aninspection of the accompanying drawings, in which-- Figure 1 shows oneform of the invention in which the positive wave peaks are limited to apredetermined amplitude, and contribute a limit- The upper circuit asshown in the figure comprising rectifier l4 and its associated filteringelements functions as in present ground noise reduction systems toprovide an envelope voltage across the condenser l8 which is derivedentirely from the negative peaksof the input audio voltage. Thus, ifcondenser 21 were short-circuited, the system would operate exactly asin present ground noise reduction systems. The overall response of thesystem can be so adjusted that the displacements of the shutter are justsufiicient to clear the tops of the waves in the recording systern, butit is practically necessary to provide 'an excess of gain by means inthe audio frequency-system which supplies the input to transformer ID.This margin of exapproximately proportional to the magnitude of themodulation and is thus comparatively small in absolute magnitude whenthe modulation 'is low, while at higher amplitudes itbecomes-unnecessarily large. In order to provide a morenearly constantmargin, it has been proposed to so load the rectifier or the output ofthe audio transformer which is applied to the rectifier in a systemotherwise identical with the upper circuit of modulations, the systemhas. a relatively high gain, and, at higher modulations, the gain isreed amount to the total control voltage which determines theresponse ofthe system.

Figure 2 shows a second form of the invention in which the wave peaks ofboth polarities, in a portion of the circuit, are limited to apredetermined amplitude and applied to the circuit in a difierent mannerfrom that shown in Figure 1, for providing a margin of response abovethat produced by the unlimited negative peaks.

Figure 3 shows a modification of the invention in which the limitingvoltage applied to the positive wave peaks can be raised duringcrescendos.

Figure 4 shows a modification of the form of the invention shown incharge or recovery of the system, and

Figure 5 shows a form of the invention in which the.voltage applied bythe positive peaks is controlled by the voltage built up by the negativepeaks.

In Fig. 1, the audio input to the ground noise reduction system isapplied winding of transformer ID. This transformer duced. In a patentapplication of Glenn L. Dimmick, U. S. Serial No. 358,813, filedSeptember 28, 1940, entitled, Film sqund recorders, and assigned toRadio Corporation of America, this is accomplished by means of arectifier element and biasing battery similar to the one shown in Figure3 in which a photocell is used for controlling the rate of disto the prmary.

is provided with. two secondary windings It and I! each ofwhich operatesrectifiers l4 and 23, respectively. The outputs of the rectifiers areindependently filtered, that from rectifier M being filtered bycondensers l6 and IB in cooperation with resistance I1, and the outputfrom the rectifier 23 is filtered by condensers 25 and 21 in cooperationwith resistance 26. The final output after filtering consists in thevoltage across conthe original sound waves and partly because of densers21 and [8 in series and is thus strictly additive. After modulationceases, condenser Hi discharges through resistance l5 while condenser 25discharges through resistance 24,- thus providing for recovery ofthrough the winding of shutter'28.

the circuit to normal zero, modulation condition; The summation voltagevention is that, in my Fig. 1, the rectifier I4 operating on thenegative peaks is adjusted for bare clearance and the margin isderivedfrom the positive peaks which operate through rectifier 2L- Theadvantage of employing the positive peaks to produce the margin hasalready been described. In order Y that the margin may be achieved forvery low amplitudes, the winding l2 of transformer 10 more turns thanwinding H so that low voltages applied to the primary will producerelatively to apply across the rectifier high output voltages Y 23. Wereno limiting device applied to the margin-producing circuit of Fig. 1, itwould cause undesirable large response on the part of the shutter,partly because the positive peaks are likely to be higher than thenegative peaks in the greater number of turns on windingl2. I thereforeemploy the limiting device-comprising rectifier 2| and biasing battery22. Whenever a positive peak occurs,- it tends to send current throughrectifier 23. It will also send current through rectifier 2!, providedthat voltage produced exceeds that-of the 'bias battery 22. If the peakvoltage is less than the voltage of the bias battery, no current willflow through rectifier 2|. Rectifier 2l is of such low resistance thatit constitutes a heavy load on the audio circuit which comprises thesecondary winding l2 v of a volume control Fig. 1 that, for low ispreferably provided with diiferent manner in Fig. 24than in Fig. 1. Fig.2, it is applied to the rectified voltage, whereof transformer l 0. Inorder that the loading may be more efl'ective in limiting the voltageapplied to rectifier 23, the resistance of the circuit is furtherincreased by including a resistor 3|, the resistor 3| also serving thepurpose of prevent ing the loading from reacting as much on the voltageproduced in winding l,l, although this is not an essential feature sincethe loading affects only the positive peaks and not the negative Thepurpose of resistors l3 and 20 is to cause the audio circuits to have alow effective output resistance particularly to minimize the efiect ofleakage reactance in the transformer and which might otherwise prevent asudden fiow of current through the rectifiers when this is needed. Sincethe peaks applied to rectifier 23 are limited to a voltage"substantially equal to that of bias battery 22, the entire lower ormarginal circuit can produce only a certain limited voltage. This tendsto give the desired constant margin.

While there is more to be gained by producing the margin by means of thepositive peaks than ofthe negative peaks, there is no reason why bothshould not contribute to the margin. There is, in fact, the possibilitythat the early peaks in r as in Fig. 1 the limit was imposed on thevoltage before it was applied to the rectifier element.

This difierence is not essential to the perform would tend to reduce themargins. This will never do any harm; since small margins duringdiminishing audio voltage are desirable. In any case, the reduction ofmargin during diminuendo would be less than the increase duringcrescendofor the reason thatthe filter circuits are so designed that the recoveryof the circuit when modulation is decreasing is always smaller than theopening during the equally rapid crescendo.

In Dimmick Patent No. 2,222,172 of Nov. 19,. i

1940, it has been further proposed to control the recovery of thecircuit or restoration to zero modulation condition by, dischargin thecondenser l6. through a photocell instead of a simple resistor. After anaudio voltage peak has occurred, it is not desirable that the condenser16 of Figures 1 to 4 shall begin at once to discharge at the maximumrate. The production-of -a smooth envelope current is favored bydelaying the discharge so that it reaches full rate only after themodulation has fallen considerably.

Dimmick therefore proposes .to control the passage of discharge currentthrough the photocell by varying the illumination of the cathode of thecell by means of a lamp whose current is controlled in a suitablemanner. Theinarginal circuit shown inmyl figures 1 to 3 providesa.eonvenient source of voltage for controlling the illumination producedby a lamp such as disclosed in the Dimmick application. Figure 4 shows amodification of my circuits comprising the photocell discharge feature.In nearly all respects, Figure 4 is the same as Figure 3 with the,exception that ance of the system and is'optional with the de signer ofthe circuit. In the case ,where a full wave rectifier is .used,simplification is served by audio voltage. The resistor 3| serves thesame loading the rectified voltage rather than the purpose as in Fig. 1,but is more important in that the negative peaks are loaded by thelimiter in Fig. 2 and would; therefore, were it not for the protectiveeffect of resistance 3|, also cause a reduction in the height of thenegative peaks developed in transformer winding ii.

In U. S..Patent No. 2,222,172 of Nov. 19, 1940, Glenn L. Dimmick hasdisclosed means for pro viding a control voltage in a ground noisereduction system which voltage is proportional to the rate of change ofgrid voltage applied to an out-- put tube, or, in other-words, equal tothe rate of change of the shutter current... This voltage resistance I5is replaced by photocell 42 which in turn is illuminated by lamp ll. Thebrightness of lamp 4| is controlled by electrondisa charge tube 40 whosegrid or control yoltage'is derived from the marginal rectifien. The gridof the tube is so connected that, when there is mod ulation present andan appreciable rectified voltage produced by 'rectifiers 23,- thenegative bias on the grid of electron discharge tube 40 is. in-'creased, thereby reducingthe lamp current.-

7 .Thus, discharge of condenser I6 is retarded by low illumination ofthe photocell so long as there is appreciable modulation aflfectingjhejmar'gin circuit voltage. With the margin circuit exactly as in Figure3, the expedient justdescribed might is high-during rapid crescendosandat,other times is much less. Figure 3 .shows a circuit adapted to makeuse of such a voltage for increasing the margin of shutter openingduring periods of rising amplitude. Figure 3 'difiers from Figure 2 onlyin. the inclusion of the secondary winding of transformer 32 in srieswith the biasing battery'22. The limiting voltage supplied by themarginal circuit in Fig. 3 will therefore be dependent not solely on thebias battery 22 but on the sum of battery voltageand the voltage inducedin the secondary oftransformer 32. The;

number of 'tu'ms on the secondary winding is ad-.

justed to give the desired degree of increase in.

margins during crescendos. During periods of diminishing audio voltage,the voltage provided bytransformer 32 is in the reverse direction andtend to produce too muchof an oifand on effect or cause the lamp to beeither brightly illuminated or dark. It would normally be; advantageousto have the brightness of the lamp 4i pr ogressively decreased by themagnitude of the audio signal rather than to have an off-on effect asalready mentioned. This can'be readilyaccomplished' without impairingthe sharply limitvoltage' ing effect on the circuit on the rectifiedemployed for margin,- by addinga resistance 32 through which therectified current provided by the rectifier 23 must pass before reachingthe loading circuit 2l22.' The voltage applied to the grid of dischargetube 40 is thus the sum of the limited voltage and a voltagewhichcontinues to increase as the modulation rises, namely, the

resistance drop across' resistance :element 32.

-With this arrangement, photocelljlis. substantially nonconducting,while theregis. high modulation, and, as the modulation falls, it tendsto carry morec'urrent, thus permitting sufliciently rapid recovery ofthe circuit after the cessation of modulation.

"Figure 5 shows a modification or my invention in which the marginproduced by the-positive half waves is addedto the ground noisereduction system" responseas produced in the nor-r the negative peakscan cause an unnecessarily large margin, is obviated in my Figure 5 bylimiting the magnitude of the positive peaks to a value dependent on themagnitude of the negav tive peaks. In Figure 5, winding II, resistor I3, rectifier l4, discharge-resistance IS, the filter comprising elementsIS, IT, and I8, and the output tube l9 function in thesame manner as inFigure 1. Also contributing to the charging of condenser are thepositive peaks developed in winding 55 of transformer, I0 and applied tocondenser IS through rectifier 23. Were the resistance .31 and loadingrectifier 2l omitted in Figure 5, and windings 55 and H provided thesame number of turns, Figure 5 would operate as an ordinary or' standardground noise reduction circuit except that it employed full waverectification of the usual type. This, as has been previously explained,is open to the objection of [producing too much margin because of thegreater height of the positive waves. Provision istherefore made in Fig.5 for limiting the positive waves. more easily applied and efiective,the audio circuit for rectifying the positive peak is made of highimpedance by the inclusion of resistance 3!. The magnitude of thepositive peaks applied to the rectifier 23 is limited by a secondrectifier 2! in series with a source of biasing voltage derived in a-manner presently to be explained. The loading system operates in thesame manner as that in Fig, land limits the peaks to a magnitudesubstantially equal to the biasing voltage, which in 1 was supplied by abattery and, therefore, constant, but in Fig. 5 is dependent on themagnitude of the negative peaks. The bias voltage may be made exactlyequal to the voltage developed by the negative peaks across condenserIE, but, for purposes of illustration, I have indicated means forcausing the bias voltage to slightly exceed that produced by thenegative waves. This permits the positive waves to provide a marginwhich is useful in preventing clipping. In Fig. 1, the bias voltagewhich-prevents current passing through rectifier 2l, unless the appliedvoltage exceeds the bias voltage, was a simple battery and of very lowresistance. It is necessary in order that the loading circuit shall beeffective that it operate into a low impedance circuit. This lowimpedance circuit is provided by the relatively large condenser 52 whichrdischarges when modulation ceases through the correspondingly lowresistance 54. If condenser 52 were charged by means of negative peaksof the' same magnitude as those which operate through rectifier M tocharge condenser l6, then the positive peaks would be limited to thesame magnitude as the negative peaks. In order to provide a margin, Ihave included additional turns 50 on the secondary of transformer 1!],such that the total. audio voltage for providing the bias is thatdeveloped in windings H and 50 in series. The resistor 51 serves thesame purpose with reference to winding 50 as the resistor 13 inreference to winding H, namely, to. give the transformer secondary animpedance characteristic which will permit sudden bri-ei chargingcurrent. I have shown a battery In order to make this limit 53 which mayserve either as an alternative or supplementary means of providing thedesired slight excess of voltage across condenser 52 as compared withthat across condenser I6. It is essential that the transformer H] be sodesigned that it is capable of supplying larger instantaneous currentsthan are ordinarily required in a ground noise reduction system, for thereason that not only must condenser "5- be quickly charged when anegative peak occurs but the much larger condenser 52 must also becharged to the sameor perhaps to a higher value. From the foregoing, itwill be noted that the voltage across condenser 52 follows closely thatacross condenser IE but may be made to have either a slight fixed marginofexcess voltage resulting from the use of battery 53 or a smallpercentage margin resulting from the inclusion of the excess turns 58 inthe secondary winding of transformer l0. Since the voltage which can becontributed through rectifier 23 operating it on the positive peaks ofthe audio Waves is limited to the voltage across condenser 52, thepositive waves can never result in an increase in total responseabov-ethat desired, in View of the height of the negative peaks, but adefinite and controllable margincan be obtained. The advantage of thecircuit of Fig. 5 as compared with the circuits of Figs. 1 to 4 is thatthe inclusion of positive peaks of magnitude substantially equal to thenegative peaks in the system for charging condenser l6 results in thelatter normally receiving two charges per cycle of audio frequencyvoltage, and this results in less fluctuation in the voltage acrosscondenser IS. The fluctuations due to the periodic charging of condenserl6 and its partial discharge in between moments of charging, al-

though filtered by elements I! and iii, are sometimes a cause of audibledisturbances introduced into the sound record by the ground noisereduction system, and the danger of such noises is decreased by chargingcondenser l6 twice durin each audio cycle.

I claim as my invention: 1. A ground noise reduction system comprising ahalf wave rectifier for providing an envelope .current, a secondrectifier arranged to rectify waves of opposite polarity to those whichsa d first rectifier rectifies, means for limiting the rectified voltageof the second rectifier, and means for adding the voltages derived fromsaid rectifiers. I

2. Apparatus as defined inclaim 1, wherein the second rectifier is afull wave rectifier.

3. Apparatus as defined in cla m 1 including means for causing thelimits imposed by said.

limiting means to be increased during periods of increasing soundintensity.

4. Apparatus asdefined in claim'l including means responsive toincreases or decreases in wave amplitude for altering the limitingeffect of said limiting means. I

5. A ground noise reduction system comprising a rectifier for providinga rectified voltage substantially proportional to peak voltages ofspecified polarity of Waves to be recorded, means including a secondrectifier for providinr. a marginal voltage above the voltage of .saidfirst rectifier,

and nonlinear limiting means whereby said margin is restricted to avalue substantially less than the maximum voltage derived from saidfirst rectifier.

6. A ground noise reduction system including means for rectifyingelectrical waves of one polarity, means for rectifying electrical wavesof enemas v 5 the opposite polarity, means for adding the voltl 9. Aground noise reduction system including ages oiisaid rectified waves,and means for limitmeans for rectifying electrical waves of one poingone of said voltages to a predetermined value. larity, means forrectifying electrical waves of '7. A ground noise reduction systemincluding the opposite polarity,'mean's for adding the voltmeans forrectifying electrical waves of one 110- 5 age of said rectifiedcurrents, and means for vlarity, means for rectifying electrical wavesof limiting one of said voltages to a value notesthe opposite polarity,means for adding the voltceeding the other voltage by more than apredeages or said rectified waves, and means for limitterminedpercentage.

a ing one of said voltages to a value not exceeding 10. A ground noisereduction system includthe other voltage by more than a. limited amount.10 ing means for rectifying electrical waves of one 8. A ground noisereduction system including polarity, means for rectifying electricalwaves of mean for rectifying, electrical waves of one 110- the oppositepolarity, means for adding the voltlarity, means for rectifyingelectrical wavesv of the ages of said rectifled currents, and means forlimopposite polarei ty, means for adding the voltages iting one of saidvoltages to a value not exceedof said rectifi d currents, and means forlimiting .15 ing the other voltage by more than a predeterone of saidvoltages to a value not exceeding mined fixed amount.- the other voltageby more than a predetermined amount. EDWARD w.

